MXPA02008400A

MXPA02008400A - Application of digital processing scheme for enhanced cable television network performance.

Info

Publication number: MXPA02008400A
Application number: MXPA02008400A
Authority: MX
Inventors: Robert L Howald
Original assignee: Gen Instrument Corp
Priority date: 2000-02-29
Filing date: 2001-02-28
Publication date: 2003-01-28
Also published as: EP1260039B1; US20020010942A1; AU2001250785A1; WO2001065732A3; DE60116614T2; KR20020093818A; CA2400150A1; EP1260039A2; CN1406418A; WO2001065732A2; DE60116614D1; JP2003526254A

Abstract

A system and method for increasing the performance of a digital return path in a hydrid fiber coax television system using baseband serial optical transport, receives an analog composite return path waveform at a comparator input to a digital return transmitter that includes an A D converter and a first nonlinear processor. A first processing function is applied to signal output from the comparator at the first nonlinear processor and the processed signal is forwarded to the A D converter which converts the processed signal to generate a quantized output signal of a sequence of digital words whose value represent analog signal samples. The quantized digital signal is output to an output of the digital return transmitter and to a feedback loop including a D A converter, which converts the quantized digital signal to an analog feedback signal and forwards the analog feedback signal to a second processor. The second processor applies a second processing function to the analog feedback signal and outputs the processed analog feedback signal to the comparator input of the digital return transmitter. The comparator input to the digital return transmitter adds the processed analog feedback signal to the analog composite return path waveform to create the signal output from the comparator.

Description

APPLICATION OF DIGITAL PROCESSING SCHEME FOR IMPROVED OPERATION OF TELEVISION NETWORK BY CABLE Field of the Invention The present invention relates generally to the improvement of the operation of the hybrid fiber coaxial cable cable television network (CATV HFC) and more particularly, to the application of digital signal processing techniques for the improved operation of the HFC return path using digital return solutions.

Background of the Invention Cable television (CATV) systems of hybrid fiber coaxial cable (HFC) have evolved in two-way digital networks during the last decade. Essentially, the main end of a network transmits signals to a plurality of remote points in a first "direct" or "down" direction. The signals are transmitted from the remote points to the main end in a second "inverse" or "up" direction. In the path Inverse, transport systems, as well as payload information, have become digital in nature, evolving from linear optics that move the return spectrum of the fiber optic nodes to the digital transport systems of the band. of base for the processing centers. Figure 1 shows a block diagram of said system. The system of Figure 1 is described in greater detail in a separate description, as indicated above in the cross-reference to related applications. Essentially, the signal of the return path to the main end coming from the fiber optic node is represented by coding it completely in the form of ones and zeros. Specifically, the composite return path waveform is converted into a sequence of digital words whose value represents analog signal samples (A / D 100), the digital word is adapted in a series stream with appropriate synchronization information (Serializer / Des-serializer 110), and the digital electrical signal is converted into an optical digital signal and transmitted through fiber optics (Optics TX 120). The optical path takes the signals to the main end which has the appropriate components to receive and process the optical signals, that is, the process is inverted on the receiver side (Optics RX 130, Serializer / Des-serializer 140, D / A 150 ). The use of this digital optical technology provides many key advantages compared to traditional analog systems. Among these are the longest distance capacity, the insensitivity of operation to the length, the robustness of the environment, the cost benefits and the flexibility of the interface. The operation of digital return links can be compared favorably with their analogue counterparts. Additionally, the operation can be handled in a flexible manner against the bandwidth. This occurs, noting that less resolution bits of the Analog to Digital (A / D) converter are used to establish the signal-to-noise ratio (SNR) for the signal being transported. Fewer bits to carry means a lower SNR, but also a lower data rate. More bits they mean a higher SNR, at 6 dB / bits. As such, it is advantageous to find means to improve the SNR after the A / D conversion for lower resolution conversions. If the SNR can be increased by signal processing, a lower number of transport bits covering an SNR compared to the basic digital return system of Figure 1 can be used. Such a method is widely adapted in the technology category of noise formation. Improvements in the performance of the CATV return path using cost-effective technologies is a major problem in deploying reliable network architectures for HFCs. In this regard, there is continuous progress in the operation and speed of manufacturers of integrated circuits (IC) of converters (A / D) from analog to digital in the art. However, while it is straightforward to obtain a reasonable SNR at the receiving end with the high speed A / D technology currently available, HFC architecture designs comprise more complexity than this simple point-to-point example. In the environments of the concentrator practice and main end, it is generally the case that the inputs received from topologically diverse nodes are combined (summed RF) at the main end. Each of said combinations comprises a noise penalty of 3dB, or effectively decreases the resolution of the A / D system by one half of a bit. In essence, a system designed with a 10-bit A / D converter in the field, and combined four times at the main end, has the theoretical operation of an 8-bit system. Similarly, if it is desired that the operation of the end of the line have 10 bits of resolution after a combination of four paths, then the conversion process must start with 12 bits of theoretical operation in each node. Therefore, the present invention is focused on improving the performance of the return path of the CATV HFC baseband optical digital transmission using cost effective digital solutions.

Summary of the Invention a system to improve the functioning of the HFC return trajectory according to the present invention, implements a DSP method to increase the signal to noise ratio (SNR), thereby improving the operation of the HFC return path without resorting to higher resolution A / D converters. The method uses well-known signal processing architectures, applied to an RF system to achieve a reduction in the band noise quantification. The technique is applied to any HFC return architecture that uses a digital baseband optical transmission in the implementation of the inverse path. An exemplary embodiment of the present invention includes a system and method for increasing the operation of a digital return path in a hybrid fiber coaxial cable television system that uses an optical transport of a baseband in series, receives a form of Analog composite return path wave at a comparator input, and a digital return transmitter including an A / D converter and a first non-linear processor. A first processing function applied to a signal output from the comparator in the first non-linear processor, and the processed signal is sent to the A / D converter which converts the processed signal to generate a quantized output signal of a sequence of digital words whose value represents analog signal samples. The quantized digital signal is produced at an output of the digital return transmitter and for a feedback circuit that includes a D / A converter, which converts the quantized digital signal into an analog feedback signal and sends the analog feedback signal to a second processor. The second processor applies a second processing function to the analog feedback signal and produces a processed analog feedback signal for the comparator input of the digital return transmitter. The comparator input to the digital return transmitter adds the analog feedback signal processed to the waveform of the analog composite return path to create a signal output from the comparator. In an embodiment of the present invention, the method further includes the step of filtering by pass under the quantized digital signal and still a additional mode, the step of testing the filtered quantized digital signal.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and other additional objects, features and advantages of the present invention, may be appreciated from the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 1, illustrates the basic elements of a hybrid fiber coaxial cable digital return path transport system. Figure 2 illustrates a simplified block diagram of a digital return transmitter with a non-linear processor. Figure 3 illustrates the noise quantization spectrum of an A / D converter with an input similar to noise. Figure 4 illustrates the quantization spectrum of the noise formed against the quantization spectrum of the raw noise.

Figure 5 is a graph of rms noise against the curve smoothing index and illustrates the effect of non-linear feedback in the unification of noise quantification. Figure 6 illustrates a simplified block diagram of a first sigma delta A / D converter that includes a first-order modulator, and a digital declinator.

Detailed Description of the Invention The basic elements of the proposed system for the improved performance of the return path for digital communication signals are shown in Figure 2, which shows an example topology of an A / D converter with functional block diagrams additional, which perform the digital signal processing (DSP) algorithms designed to improve the SNR compared to a system that does not perform the processing functions. A non-linear method is implemented in the DSP, used in high performance applications, such as the audio processing that creates this improvement. The system shown in Figure 2 illustrates a processor and an A / D converter in which an analog input signal A (s) is input to a comparator 10, whose output is coupled with a non-linear processor H (s) 20, whose output is coupled to an A / D converter 30. In a feedback circuit, the output of the A / D converter 30 is coupled to a D / A converter 40 and to the processor F (s) ) 50 in the feedback circuit to the input comparator 10. Essentially, the input - to the circuit is fed to the quantizer by means of the non-linear processor, and the quantized output is fed back through the D / A converter 40, which converts each sample of the digital signal to generate an analog feedback signal, which is coupled and subtracted from the output, forcing the average value of the quantized signal to track the average input. Those skilled in the art will appreciate that the implementation of this type of converter is generally based on low frequency implementations, such as for high fidelity audio. Additionally, the level of quantification is generally of lower resolution due to the ability to implement the DSP more effectively at the lower speeds generally used. For the CATV application, the implementation of the DSP algorithm is increased significantly in its complexity and design due to the nature of the necessary high-speed processing. Referring to Figures 3 and 4, the operation of the processor and the nature of the improvement provided by the implementation of the converter in the digital return transmitter is explained, which is shown graphically. The noise quantization spectrum of the output of an A / D converter is shown in Figure 3. The bandwidth of the inverse path is assumed to be identical to the Nyquist bandwidth at the Bnl output of the A / D converter. As can be seen, the noise spectrum is modeled as a plane over the Nyquist bandwidth of the converter. Figure 3 shows a proven output spectrum with typical relationships between the three previous parameters. The higher clock frequency in relation to the reverse path bandwidth Bn ?, converts the spectrum density lower, providing a means to decrease the noise power in the reverse bandwidth. Figure 3 shows this example, where the clock frequency is increased, which goes from Bn? to Bn2, and decreasing the density of the spectrum. In other words, the same amount of noise power, determined by the resolution of the A / D converter, is dispersed in a wider Nyquist bandwidth. This "curve smoothing" technique illustrated with reference to Figure 3 is an inefficient way to obtain a noise reduction, since the clock rate must be doubled to achieve only a 3 dB improvement of the SNR. Additionally, the digital inverse system for HFC, is already implemented in the A / D converters that are operated in the current technique of clock indices. In commercial apparatuses it is not possible to increase the clock rate enough for greater operating gains, without suffering severe degradation or making the part completely non-functional.

Instead of relying on curve smoothing just to provide the increase in the SNR, Figure 2 shows a diagram of a nonlinear processor, H (s) which implements a transfer function, which produces this capability. Also, the F (s) of the feedback circuit can provide a filtering additional as needed. Both the H (s) and the F (s) can take many topologies, depending on the desired improvement and the complexity of the implementation. However, the non-linear nature makes accurate analysis difficult, especially when higher-order architectures are used. In many cases the behavior can only be characterized through simulations. A noise spectrum of said resulting processor is shown as an example in Figure 4. In this case, the spectrum which previously had a uniform density (white), for Bn, is no longer flat. The power of the noise between the uniform density and the non-uniform density is the same, but in the latter case, the power is changed in the region of the spectrum between Bn? and Bn2. That is, the modulator 'forms' the quantization noise so that most of the energy will be above the bandwidth of the signal. Since the bandwidth of the inverse system is Bn ?, the region that now contains most of the noise power can be filtered without effect on the desired signal. Now much of the noise has been changed to this region, the noise power within the signaling band has been reduced. This reduction in noise power is equivalent to the effect of using a higher resolution A / D converter in that region of the tested spectrum. Because the region of the tested spectrum in which noise reduction occurs is only the one that concerns us, this technique is essentially for an increase of an effective bit proportional to the fall in noise power within Bn? . In a representative example of what can be expected, SNR improvements of 20 dB can be achieved, which correspond to more than three bits of additional resolution. The exact gain depends largely on the amount of curve smoothing and non-linear processor architecture.

Mathematically, the analysis of noise reduction can be expressed, in the simplest case, using figure 2 as a guide. Suppose that F (s) = I. For the quantized output Y (s), the quantized noise Q (s), the analog input A (s), and the transfer function of the low-pass processing, high gain H (s) ), the diagram shows the following: Y (s) = [A (s) - Y (s)] H (s) + Q (s) (1) Y (s) + Y (s) H (s) = A (s) H (s) + Q (s) (2) Y (s) =. { A (s) H (s) / [1 + H (s)]} +. { Q (s) / [1 + H (s)]} (3) Then, assuming that the bandwidth of interest of the input that | H (s) | > > 1, (3) becomes Y (s) = A (s) + Q (s) / H (s) (4) Because | H (s) [»1, the last term may be small. The density of the noise quantization spectrum, Q (s), is reduced by the magnitude of H (s) of the signal bandwidth. However, outside the bandwidth of the signal the density of the spectrum is increased. Of course, this part of the spectrum is not of interest. However, in order to make correct use of the change of spectrum energy at the high end of the band, digital filtering is done after the quantization process to reduce the noise power (eg, the increased noise during the conversion process can be controlled in a very effective way). As illustrated in Figure 6, which shows a first sigma delta A / D converter coupled to a digital decimator, a low pass digital filtering step (low pass filter 70) can be implemented to smooth the output of the digital modulator, significantly attenuating the out-band quantization noise, interference, and high-frequency components of the signal. Also, if desired, the descending test (descending tester 80) can be implemented to bring the displayed signal to the Nyquist index. As an example, consider an 8-bit A / D converter. This number is chosen because it has a practical implication in terms of both the operation and speed of the HFC application. At present, an 8-bit apparatus off the good shelf, tests approximately 200 MHz, providing approximately a two-fold curve smoothing for the HFC return paths. Within another year, the high end parts now available that go faster, will be available in volume and at a low cost, suitable for CATV applications. A smoothing of four times curves will be easily found within range. Figure 5 is a graph of band noise versus the curve smoothing index for the PCM examples and one, two and three feedback circuits. Referring to Figure 5, it can be seen that a four-fold curve smoothing with a second-order feedback system provides approximately 20 dB of reduction in quantification of additional band noise. At 6 dB / bits, this represents more than three bits of effective resolution, changing a 7.5-bit conversion process (a non-ideal 8-bit A / D converter) to almost 11 bits of effective resolution. This represents an essentially better performance than any similar return technology in widespread use today, such as linear DFB laser transmitters. Additionally, unlike the case of linear optics, the operation is independent of distance. In terms of implementation, architectures such as the previous one where F (s) = l, which represents a sigma delta modulation, are well suited for digital design technologies, such as FPGA and custom ASIC design. However, the required clock rates, necessary to achieve the curve smoothing index, are relatively high for the commercial implementation of the FPGA in today's technology. IC design developments have created chips with capacity for these processing indices. It is anticipated that the commercial obstacle of the FPGA will also be overcome in the near future, as the development continues. The above describes a DSP system for increasing the performance of the HFC return path without resorting to the higher resolution A / D converters. The method uses well-known signal processing architectures applied to an RF system to achieve a reduction in the band noise quantification. The individual components are known and widely available. The technique is applicable to any HFC return architecture which uses a digital baseband optical transmission in the implementation of the inverse path. Although various modalities are illustrated and described in the present description in a specific manner, it will be appreciated that the modifications and variations of the invention are covered by the foregoing teachings, and that they fall within the scope of the appended claims without departing from the spirit and intended scope of the invention. For example, referring again to Figure 2, the processor F (s), like the processor H (s), can take any variety of transfer function responses to meet the application's operation requirements. Furthermore, the example modification should not be construed as limiting the modifications and variations of the invention covered by the claims, but are merely illustrative of the possible variations

Claims

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following claims is claimed as property: CLAIMS 1. A method to increase the functioning of the digital return path in a television system of hybrid fiber coaxial cable using the optical transport of baseband in series, wherein the method is carried out in an optical fiber node, said method comprising the steps of: receiving a composite return path waveform analogous in an input of a comparator to a digital return transmitter that includes an A / D converter and a first non-linear processor; applying a first processing function to the signal output of the comparator in the first non-linear processor and sending the processed signal to the A / D converter; convert the processed signal to generate a quantized output signal of a sequence of digital words whose value represents analog signal samples; producing the quantized digital signal at an output of the digital return transmitter, and to a feedback circuit including a D / A converter; converting the quantized digital signal into a similar feedback signal and sending the analog feedback signal to a second processor; and applying a second processing function to the analog feedback signal and producing the analog feedback signal processed to the digital return transmitter comparator input, wherein the comparator input to the digital return transmitter adds the processed analog feedback signal to the waveform of the composite analogue return path to create the signal output of the comparator.
2. A method according to claim 1, which comprises the steps of: adapting the quantized output of the digital words in a stream in series with the appropriate synchronization information to identify the boundaries between the words and retrieve the timing of the bits themselves; convert the digital electrical signal into an optical digital signal and transmit the optical ones and zeros by the optical fiber; and reverse the previous steps on the receiving side.
3. A method according to claim 1, which further comprises the passage of the low pass filtering of the quantized digital signal.
4. A method according to claim 3, which further comprises the step of testing the filtered quantized digital signal in a downward manner.
5. A method according to claim 4, wherein the quantized filtered digital signal is tested in descending order at a Nyquist index.
6. A method according to claim 1, wherein the second processing function is equal to the first, and the digital return transmitter represents a sigma delta modulator.
7. A method according to claim 6, wherein an average value of the quantized digital signal tracks an average processed analog feedback signal.
8. A system for improving the operation of a digital return path in a hybrid fiber coaxial cable television system using optical transport of the baseband in series and comprising: comparison means for comparing a waveform of analog composite return path received in a digital return transmitter with a feedback signal from the digital return transmitter including an A / D converter and a first non-linear processor; means for applying a first processing function to a signal output of the comparison means in the first non-linear processor and sending the processed signal to the A / D converter; means of the A / D converter for converting the processed signal to generate a quantized output signal of a sequence of digital words whose value represents analog signal samples; production means for producing the quantized digital signal at an output of the digital return transmitter and to a feedback circuit including a D / A converter; convert from digital to analog the quantized digital signal to a similar feedback signal and send the analog feedback signal to a second processor; and means for applying a second processing function to the analog feedback signal and producing the analog feedback signal processed at the comparator input of the digital return transmitter, wherein the comparison means adds the processed analog feedback signal to the analogue feedback signal. waveform of the analog compound return path to create the signal output of the comparison means.
9. A system according to claim 8, further comprising: serializer means for adapting the quantized output of the digital words in a stream in series with an appropriate synchronization information to identify the limits between the words and recover the timing of the bits themselves; optical transmission means for converting the digital electrical signal into an optical digital signal, and transmitting the optical ones and zeros by an optical fiber; and means for reversing the previous steps on the receiving side.
10. A system according to claim 8, which further comprises filter means for filtering the quantized digital signal by low pass.
11. A system according to claim 10, further comprising downstream test means for downwardly testing the filtered quantized digital signal.
12. A system according to claim 11, wherein the filtered quantized digital signal is tested in descending order at the Nyquist index.
13. A system according to claim 8, wherein the second processing function is equal to the first and the Digital return transmitter represents a delta sigma modulator.
14. A system according to claim 8, wherein an average value of the quantized digital signal tracks an average processed analog feedback signal.
15. A method for improving the signal-to-noise ratio (SNR) in the return path of a two-way CATV HFC system using serial baseband optical transport comprising the steps of: receiving a waveform of Analog composite return path in an input to a digital return transmitter that includes an A / D converter and a non-linear processor; apply a digital signal processing algorithm to the input signal in the non-linear processor and send the processed signal to the A / D converter; converting the processed signal into the A / D converter to generate a quantized output signal, wherein the SNR of the quantized output signal is greater than the SNR of the signal that was not subjected to the digital signal processing algorithm.
16. A method according to claim 15, further comprising the steps of: converting the quantized digital signal into an analog feedback signal and sending the analog feedback signal to a second processor; and applying a second processing function to the analog feedback signal and producing the analog feedback signal processed at the input of the digital return transmitter. where the entrance to the transmitter. digital return adds the analog feedback signal processed to the waveform of the analog composite return path.
17. A method according to claim 15, which further comprises the step of filtering by low pass the quantized output signal.
18. A method according to claim 17, further comprising the step of testing the filtered quantized output signal in a downward manner.
19. A method according to claim 18, wherein the digital signal filtered quantized is tested down to the Nyquist index.
20. A method according to claim 16, wherein an average value of the quantized output signal tracks an average processed analog feedback signal. SUMMARY A system and method for increasing the performance of a digital return path in a hybrid fiber coaxial cable television system using optical baseband serial transport, receives a composite return path waveform analogous to an input of a comparator to a digital return transmitter that includes an A / D converter and a first non-linear processor. A first processing function applied to the comparator signal output in the first non-linear processor, and the processed signal is sent to the A / D converter which converts the processed signal to generate a quantized output signal of a digital word sequence whose value represents samples of analogous signal. The quantized digital signal is produced at an output of the digital return transmitter and the feedback circuit that includes a D / A converter which converts the quantized digital signal into a similar feedback signal and sends the analog feedback signal to a second processor . He The second processor applies a second processing function to the analog feedback signal and produces a processed analog feedback signal at the digital return transmitter comparator input. The comparator input to the digital return transmitter adds the processed analog feedback signal to the analog compound return path waveform to create a comparator signal output. oz / 9 °°